Atherosclerosis is increasingly regarded as a chronic inflammatory process. The coagulation factor fibrinogen (FGN) provides a critical provisional matrix at sites of injury in which cells can migrate and carry out the specialized functions involved in hemostasis and inflammation. Epidemiologic, histologic, and experimental lines of evidence indicate the FGN may plan pathogenic roles in atherosclerosis. While elevated plasma FGN levels are associated with an increased risk of cardiovascular disease, it remains unclear whether RGN is a surrogate marker or directly participates in atherogenesis. Despite the importance of FGN in health and disease, the mechanism and site of the major FGN catabolic pathway(s) are unknown. Preliminary data demonstrate that the integrins Mac-1 (CD11b/CD18) on monocytes and avb3 (CD51/CD61) on hepatocytes can bind, internalize, and degrade FGN via lysosomal cathepsin D. Data obtained since the prior submission provide evidence in a rabbit model of vascular injury that interactions between FGN and Mac-1 directly influence vascular repair. The central hypotheses of this proposal are that integrins are a major determinant of FGN level, that their function in this regard is regulated by pro-inflammatory cytokines and the urokinase receptor (microns PAR), and that modulation of plasma FGN level will directly affect the development of atherosclerosis. Three specific aims are proposed: (1) to investigate the mechanism and regulation of integrin-mediated FGN turnover; we proposed to define the structural determinants important to the interaction between FGN and its integrin receptors and investigate the effect of cytokines on FGN degradation. Microns PAR appears to be an important regulator of integrins. The molecular pathways which connect integrin function with that of microns PAR will be dissected in eukaryotic cells transfected with wild-type and mutant forms of MAC-1 and avb3; (2) to define the in vivo mechanism of FGN catabolism. Plasma clearance experiments will be performed to determine the kinetics of FGN metabolism in wild-type, receptor (MAC-1 and microns PAR)- deficient; and cathepsin D minus deficient mice. An isolated perfused rat liver preparation will be utilized to investigate the effects of cytokines in FGN metabolism and to identify the cell type(s) responsible for FGN catabolism; and (3) to study the effect of FGN deficiency on the development of atherosclerosis in transgenic mice over-expressing human apoB. Homozygous FGN-deficient mice will be bred with mice expressing human apoB to generate double transgenics that express the apoB transgene and are either homozygous or heterozygous for FGN deficiency. Diet-induced atherosclerotic lesion area will be quantified over time to determine whether FGN deficiency affords protection in a murine model of atherogenesis. Collectively, these experiments are designed to clarify whether FGN has an etiologic role in atherosclerosis and, if so, provide a potential molecular basis for drug design aimed at the modulation of FGN levels.